Abstract
Studies were conducted on the sexuality of blast fungi using 78 isolates from rice and 272 isolates from 28 other species of hosts. Ascosporic isolates from crosses segregated into two compatibility groups, designated A and a, and perithecia were produced only when isolates of opposite compatibility groups were mated with each other. Pyricularia is heterothallic. The two mating types were found at the same time in all three different areas of Japan investigated. Great variability was observed in fertility among isolates from different hosts, i.e., all isolates from finger millet and 14 other hosts produced perithecia in some crosses, while none of the isolates from Panicum bisulcatum and 9 other hosts showed fertility in any of the crosses conducted. On the other hand, in crabgrass isolates only a few isolates were fertile. Light colored mutant lines which were derived from a goosegrass isolate exhibited an extremely low fertility when crossed with other mutants, whereas a large number of perithecia were produced in crosses with wild-type isolates. The culture color characteristic and the mating type were inherited independently. The mating type of the rice blast fungus was determined by crossing with the most fertile tester lines from goosegrass. It was apparent that both mating types were present in rice isolates too. However, when rice isolates were crossed with each other, no perithecia were produced. Perithecia from crosses of rice isolates with goosegrass isolates usually produced only a few normal ascospores. This suggests that there may be a genetic block preventing ascospore formation. The results of mating reaction among these related species of Pyricularia may serve as one of the criteria for classifying these species.
